Field of Invention
The present invention relates to a device for uncoupling a carrier for a bearing of a rotary shaft in a turbomachine. A carrier such as this is able to break its connection with the turbomachine stator upon the onset of imbalance in order to avoid damage to the turbomachine.
Description of the Related Art
A turbomachine comprises, from upstream to downstream in the direction in which the gases flow, a compressor, a combustion chamber and a turbine. The purpose of the compressor is to raise the pressure of the air supplied to the combustion chamber. The purpose of the turbine is to tap off some of the pressure energy of the hot gases leaving the combustion chamber and convert it into mechanical energy to drive the rotation of the compressor.
For that purpose, the compressor and the turbine are made of a first set of fixed components that make up the stator and of a second set of components capable of being rotated relative to the stator and which make up the rotor.
The compressor rotor and the turbine rotor form an assembly which is securely connected by a rotary shaft. Rotation of the rotor with respect to the stator is rendered possible by means of bearings, a bearing being a mechanical component that supports and guides a rotor, particularly the shaft of this rotor. This bearing comprises a first part fixed to the rotor shaft and a second part fixed to the stator via a bearing carrier. A rolling bearing assembly is positioned between the two parts of the bearing thus allowing one part of the bearing to rotate relative to the other. The rolling bearing assembly may, for example, be of the ball bearing, cylindrical roller bearing, or taper roller bearing type.
A turbomachine may also be of the “twin-spool” type, which means that it has two rotors arranged coaxially, a bearing allowing relative rotation of one of these two rotors with respect to the other.
A turbomachine may also comprise a fan, that constitutes the first stage of the compressor. The fan has very large blades known as fan blades, which increase the mass and inertia of the rotor.
If a fan blade breaks, imbalance appears on the shaft supporting the fan. Imbalance is a phenomenon that affects the balance of the rotor, the center of gravity of which is no longer precisely on the axis of rotation as it should be. Cyclic loadings and substantial vibrations are therefore imparted to the turbomachine stator, via the bearing carrier, with a great risk of damage that could lead to self-destruction. In order to prevent these undesirable phenomena from being transmitted to the stator, it is necessary to uncouple the bearing carrier, that is to say to interrupt the mechanical transmission of rotation, notably by disconnecting the two parts that form the bearing carrier.
Document FR 2877046 describes a solution that consists in using bolted connections that can rupture in order to attach an upstream part and a downstream part that form a bearing carrier. The rupture screw of each bolted connection passes through an upstream hole of an upstream part and a downstream hole of a downstream part of a bearing carrier, the downstream part of the bearing carrier forming an integral part of the casing. The screw head of the rupture screw is adjacent to the hole of the upstream part and is in contact with this upstream part on a plane perpendicular to the axis of the hole. The portion of the rupture screw that passes through the hole is in contact with the inside of the hole via a centering portion and has a portion of reduced cross section liable to rupture when a predetermined tensile force is exceeded, thus uncoupling the two parts that make up the bearing carrier.
It will also be noted that with this type of bolted connection that can rupture, the longitudinal positioning of the low-pressure compressor shaft can be achieved via a thrust bearing, in the form for example of a ball bearing, between the drive shaft and the upstream part of the bearing carrier.
However, with such a rupture screw, when imbalance appears, the upstream part and the downstream part move relative to one another in a circular relative motion which has the effect of subjecting the rupture screw to shear loadings, because of the tangential contact around this rupture screw, and these may lead to uncontrolled rupturing of the rupture screws. Now, these rupture screws are designed for tensile loadings, and this has a deleterious effect on the uncoupling of the bearing carrier.
In order to improve control over the uncoupling function, document EP 2071 138 describes a solution which involves replacing the centering portion described in document FR 2877046 with a means referred to as a “dual-centering” means. This means may in practice consist of the collaboration of a groove and of a rib of complementing shapes, in contact with one another via their two flanks, thus offering two parallel contact surfaces. Such centering, by means of these two parallel surfaces, makes it possible to reduce, if not to eliminate entirely, the ovalizing deformation of the bearing carrier by maintaining permanent contact between the flanks of the groove and of the rib. In order to reduce further, if not to eliminate, the shear forces applied to the rupture screws, document EP 2071 138 also proposes eliminating any contact between each upstream hole and the rupture screw passing through it, allowing said rupture screws to be subjected only to tensile loadings, thus guaranteeing better control over the uncoupling of the bearing carrier.
However, the high axial thrust caused by the aerodynamic forces internal to the turbomachine dictates a need for a large-sized thrust bearing. Significant bulkiness generated by this thrust bearing means that said thrust bearing has to be installed on the downstream side of the bearing carrier, while a roller bearing is installed on the upstream side of said bearing carrier.
As a result, if blades are lost, the low-pressure compressor shaft is still held longitudinally by the thrust bearing of the downstream part of the bearing carrier. As the upstream bearing secured to the shaft is furthermore a roller bearing, no forward movement drives the upstream part of the bearing carrier. This then results in a risk that the upstream and downstream parts of the bearing carrier might not disengage, and the consequence of this would be that the imbalance generated by the loss of blades would be transmitted in full to the structures.